1. Field of the Invention
The present invention relates to a method of production of A magnetoresistance effect device, and more particularly relates to a method of production of a magnetoresistance effect device fabricated utilizing a simple sputtering film-formation method and having an extremely high magnetoresistance ratio.
2. Description of the Related Art
In recent years, as nonvolatile memories, magnetic memory devices called “magnetoresistive random access memories (MRAMs)” have come into attention and have started entering the commercial stage. MRAMs are simple in structure, so ultra-high density integration to the gigabit level is easy. In MRAMs, the relative orientation of the magnetic moment is utilized to create the storage action. As the result, the number of possible re-writability is extremely high and the operating speed can be reduced to the nanosecond level.
FIG. 4 shows the structure of the MRAM. In the MRAM 101, 102 is a memory device, 103 a word line, and 104 a bit line. The large number of memory devices 102 are arranged at intersecting positions of the plurality of word lines 103 and plurality of bit lines 104 and are arranged in a lattice-like positional relationship. Each of the large number of memory devices 102 stores 1 bit of information.
Each memory device 102 of the MRAM 101, as shown in FIG. 5, is comprised of a magnetoresistance effect device for storing 1 bit of information, that is, a tunneling magnetoresistance (TMR) device 110, and a transistor 106 having a switching function at the intersecting position of the word line 103 and bit line 104. The main element in the memory device 102 is the TMR device 110. The basic structure of the TMR device, as shown in FIG. 6, is a three-layer structure comprised of a bottom ferromagnetic metal electrode (bottom ferromagnetic layer) 107/tunnel barrier layer 108/top ferromagnetic metal electrode (top ferromagnetic layer) 109. The TMR device 110 is therefore comprised of a pair of ferromagnetic layers 107 and 109 and a tunnel barrier layer 108 positioned between them.
In the TMR device 110, as shown in FIG. 6, the required voltage is applied across the ferromagnetic layers 107 and 109 at the two sides of the tunnel barrier layer 108 to cause the flow of a predetermined current. In that state, an external magnetic field is applied. When the directions of magnetization of the ferromagnetic layers 107 and 109 are parallel and the same (called the “parallel state”), the electrical resistance of the TMR device becomes the minimum ((A) state: resistance value RP), while when the directions of magnetization of the ferromagnetic layers are parallel but opposite (called the “anti-parallel state”), the electrical resistance of the TMR device becomes the maximum ((B) state: resistance value RA). Therefore, the TMR device 110 can take a parallel state and an anti-parallel state induced by an external magnetic field and store information as a change in resistance value.
To realize a practical gigabit class MRAM using the above TMR device, the difference between the resistance value RP of the “parallel state” and resistance value RA of the “anti-parallel state” has to be large. As the indicator, the magnetoresistance ratio (MR ratio) is used. The MR ratio is defined as “(RA−RP)÷RP”.
To raise the MR ratio, in the past, the electrode materials of the ferromagnetic metal electrodes (ferromagnetic layers) have been optimized, the method of production of the tunnel barrier layers have been modified, etc. For example, Japanese Patent Publication (A) No. 2003-304010 and Japanese Patent Publication (A) No. 2004-63592 propose several optimum examples of use of FexCoyBz etc. for the material of the ferromagnetic metal electrode.
The MR ratio of the TMR device disclosed in Japanese Patent Publication (A) No. 2003-304010 and Japanese Patent Publication (A) No. 2004-63592 is lower than about 70%. Further improvement of the MR ratio is necessary.
Further, recently, regarding a single crystal TMR thin film using an MgO barrier layer, there has been a report of using molecular beam epitaxy (MBE) and an ultra-high vacuum evaporation system to fabricate an Fe/MgO/Fe single crystal TMR thin film and obtain an MR ratio of 88% (Yuasa, Shinji et al., “High Tunnel Magnetoresistance at Room Temperature in Fully Epitaxial Fe/MgO/Tunnel Junctions due to Coherent Spin-Polarized Tunneling”, Nanoelectronic Institute, Japanese Journal of Applied Physics, issued Apr. 2, 2004, Vol. 43, No. 4B, p. L588-L590). This TMR thin film has a completely epitaxial single crystal structure.
Fabrication of the single crystal TMR thin film used for the single crystal MgO barrier layer described in the above publication requires use of an expensive MgO single crystal substrate. Further, epitaxial growth of an Fe film by an expensive MBE device, formation of an MgO film by ultrahigh vacuum electron beam evaporation and other sophisticated film deposition technology are required. There is the problem that the longer the film deposition time, the less suitable the process for mass production.